(1) Technical Field of the Invention
This invention relates generally to the suspension of conduits and particularly to the hanging of "strings" of piping. More specifically, the present invention relates to a casing hanger employed for suspending and externally sealing a pipe casing in a well-head. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
Casing hangers employed for suspending and sealing a casing at a well head typically comprise a generally annular shaped slip bowl and a plurality of movable slip members. The slip members have inwardly projecting teeth which engage the pipe casing centrally received in the slip bowl as the slip members are forced downwardly in the slip bowl due to the load of the casing weight. An annular ring seal is disposed below the slip members between a lower peripheral shoulder of the slip bowl and a compression ring. The seal is automatically loaded against the casing as a result of the casing forcing the slips and slip bowl against the seal.
The suspension capacity of a casing hanger is ordinarily limited by a critical threshold which is related to the suspended casing weight. When this critical threshold is exceeded an excessive casing deflection or "bottle necking" phenomena develops at the slip members/casing interface due to the compressive load exerted on the casing by the slip members. As noted above, the applied load of the casing also causes compression of the seal carried by the hanger against the casing being suspended. When the seal is placed under excessive loads, the compressive forces exerted on the casing by the seal may actually deflect or crush the casing. Thus, the seal assembly of the typical casing hanger also provides a constraint on the weight which the slip members can carry before the system fails due to a reduction of the diameter of a casing. Further, the integrity of the seal established by the casing hanger can become problematic due to dimensional variations in the casing outside diameter and variations in the concentricity of the casing. The seal assembly is ordinarily configured so that, when the seal is fully loaded, the seal assembly seals against the smallest permissible casing diameter within the given size range for which the hanger assembly has been designed. However, when the same seal is also employed for sealing against the largest diameter casing which the hanger assembly can accommodate extremely high compressive stresses may be exerted against the casing by the seal assembly.
It is, accordingly, a principal aim of the present invention to provide a new and improved casing hanger and casing suspension method which control the compressive load placed on a suspended casing by the casing engaging slip members and which also provide a sealing means for accommodating variations in casing diameter and concentricity without exerting an excessive load against the casing.
(2) Prior Art
U.S. Pat. No. 2,824,757 discloses a pipe suspension and sealing system which includes means to limit compression of the seal of a casing hanger. The hanger automatically seals with the well head in which the hanger is seated when pipe weight is applied to the hanger. A load limiting structure is incorporated into the pipe suspension assembly so that only a portion of the casing weight transferred by the slips is applied to the material which forms the seal to prevent the seal material from being placed under excess compression.
Another commercially available casing hanger employs slip members which are precision machined with sharp inner teeth. The teeth are adapted to provide a positive bite to hold the casing securely. In addition, dull outer teeth are precision machined at the back of the slip members and extend uniformly from side to side in parallel fashion for contacting the slip bowl to automatically control the slip member/slip bowl friction so that the friction increases as the casing load increases and the downward slip member travel ceases before damage to the casing can occur.